The present invention generally relates to a micro-lithographic process in semiconductor manufacturing, and more specifically to a development method in the micro-lithographic process for transferring VLSI design patterns to a wafer.
In most of the processing steps for manufacturing a semiconductor device structure, the micro-lithographic process has been used to transfer design patterns for various semiconductor layers and thin film layers. Defining areas for adding dopants also relies on the micro-lithographic process. Whether the device integration in the semiconductor industry can be moved towards a smaller line width depends on the progress of the micro-lithographic technique. Therefore, the micro-lithographic process plays a very important role in manufacturing multi-function and highly integrated semiconductor devices.
In the micro-lithographic process, essential materials include photo-mask, photo-resist, developer and cleaning solution. Photo-resists can be classified as positive and negative. The materials used in a positive photo-resist are novolak, sensitizer such as photoactive compound (PAC), additive and solvent. When a sensitizer is exposed to light, a nitrogen gas is generated and the sensitizer reacts with water to form carboxylic acid which can be resolved in an alkali solvent. Therefore, the exposed area in a photo-resist layer can be resolved in a developer. When the exposed area is removed by the developer, the design pattern is transferred to the photo-resist layer on the wafer.
A negative photo-resist contains a phenol based material. If a negative photo-resist is exposed to a light source, a nitrogen gas is generated to form an active mobile base that establishes a bridge structure to connect ring-typed resins. The active mobile base can not be resolved in a developer. Because of the swelling effect of a negative photo-resist, the resolution of transferred patterns is reduced. Therefore, positive photo-resists are usually used in the semiconductor industry in order to achieve higher resolution in the transferred patterns. In addition, a light source of a shorter wavelength has to be used to get sharper contrast and fine lines. For example, i-line with a wavelength of 365 nm is used in a 0.25 um design rule. Deep UV with a wavelength of 248 nm is used in a design rule between 0.15 um and 0.25 um.
With reference to FIGS. 1A-1E, the method of a conventional development process comprises the following steps:
1. Providing a wafer 102 seated on wafer holder 101 and coated with a layer of photo-resist 103 that have been exposed to a light source by means of a photo-mask;
2. Covering the photo-resist layer 103 with a developer 104;
3. Resolving the portion of the photo-resist layer 103 that has been exposed to the light by distributing the developer 104 through a dispenser 105;
4. Cleaning the surface of the wafer 102 with purified water 106 distributed from a water dispenser 107 to form a photo-resist pattern 109; and
5. Spinning the wafer 102 to remove water drops 108 above the wafer and drying the wafer.
The material of a photo-resist layer comprises primarily the structure of a polymer that is non-polar and hydrophobic. Because of the hydrophobic nature, the surface of the photo-resist layer is subjected to defect formation. When the surface of the wafer is patterned for contact holes, the exposed area is very small and the hydrophobic effect due to the large unexposed area becomes more severe.
FIGS. 2A-2D illustrate how a water drop 202 may cause a defect on the wafer surface. In addition to attaching a defect on the wafer 201 as a result of the hydrophobic effect, water marks are often formed on the surface when a wafer is cleaned. Because of the hydrophobic nature, the surface tension of the water drop 202 on the wafer surface is increased to condense the water into a small drop. When the water drop 202 is very small, the attraction force 204 between the wafer surface and the water drop 202 is significantly greater than the centrifugal force 203 from spinning the wafer. As shown in FIG. 2A, it is difficult to spin away the water drop 202. Impurities 206 may also be attached to the water drop 202 easily as illustrated in FIG. 2B. When the water is vaporized, the impurities 206 become water marks as shown in FIGS. 2C and 2D.
As can be seen from FIG. 2D, defects caused by water marks are usually distributed around the edge of the wafer. This is because the centrifugal force from spinning the wafer can not overcome the attraction force to remove the water drops. Although an approach to solving the problem is increasing the speed and time of spinning the wafer, the result has never been effective. In addition, this approach greatly increases the process time as well as the speed requirement of the spinning equipment.
In most of the steps of forming contact holes, the above mentioned defects due to the hydrophobic effect present a challenge to the device manufacturer. In a device using a 0.25 um design rule, the defect may cover a contact hole and results in a blind contact. As the design rule becomes 0.18 um or smaller, the defect problem becomes worse and the device yield is significantly impacted. It has been estimated that the problem may decrease the yield by more than 10%. Although it is not very effective, the semiconductor industry typically tries to solve the problem by increasing the spinning speed and time. In some cases, a TARC layer is also added to reduce the number of blind contacts caused by defects covering contact holes and to minimize the yield impact.
This invention has been made to overcome the above mentioned drawbacks of the conventional development process in manufacturing semiconductors. The primary object of the invention is to provide a development method that can reduce the number of defects introduced in the process to improve the yield of manufacturing a semiconductor device. Accordingly, a surfactant is used in the developing process to overcome the hydrophobic nature on the surface of a photo-resist layer.
Another object is to provide a development method that is effective in removing water drops and impurities on the surface of a wafer without increasing the processing time. It is also an object of the invention to provide a development method without adding more requirements to the spinning equipment used in the process.
According to this invention, a developer mixture is formed by mixing a developer with a surfactant. The developer mixture is distributed over the photo-resist layer that has a portion which has exposed to a light source with a photo-mask. In the developing step, the light-exposed photo-resist is resolved by the developer mixture and a photo-resist pattern is formed. The wafer and the photo-resist pattern are then rinsed and cleaned by purified water. A spin drying process is used to spin the wafer for removing water drops on the wafer and drying the wafer. Because the surfactant is added to the developer, the surface tension of water drops is reduced. Defects or water marks do not remain on the wafer surface since water drops are easily removed after spinning the wafer.
According to this invention, the photo-resist layer is covered with the surfactant first and then the developer is distributed over the wafer instead of mixing the surfactant and the developer in advance. In a third embodiment, the surfactant is used to cover the photo-resist pattern only after the photo-resist layer has been developed, cleaned and rinsed to form the photo-resist pattern. The wafer is then spin and dried. In a fourth embodiment, another rinsing and cleaning step is added before the wafer is spin and dried.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.